Answer:
5 months
Explanation:
The first thing to do is to obtain the difference between the two numbers of colored books, 50 and 40.
50 - 40 = 10 notebooks.
We need to calculate how long it will take for this difference to disappear, noting that Steve uses more of the red cover notebooks every month. The difference on a monthly basis is
6 - 4 = 2 notebooks per month
So, now the number of months will be gotten from;
Number of months =

Number of months =
= 5 months
We can check for this,
After 5 months, Steve would have used 5 × 6 red cover notebooks = 30
and 5 × 4 blue cover notebooks = 20
The remaining number of books will be:
RED: 50 - 30 = 20 red cover notebooks
BLUE: 40 - 20 = 20 blue cover notebooks.